1. Field of the Invention
The present invention relates to a method for manufacturing an electroluminescence element (hereinafter also called an EL element in some cases) having a light emitting layer that contains a quantum dot and is formed by photolithography.
2. Description of the Related Art
In an EL element, a hole injected from one of two electrodes facing each other and an electron injected from the other of the electrodes are coupled in a light emitting layer; a light emitting material included in the light emitting layer is excited by energy generated by the coupling; and light of a color obtained based on the light emitting material is emitted. Such EL elements have attracted attention as self-luminous planar display elements.
It is general that a light emitting layer and the like are patterned in a process of manufacturing a display device having an EL element. As the method for patterning the light emitting layer, the following methods have been proposed: a method for depositing a light emitting material through a shadow mask; a method for color-coding materials based on an ink jet scheme; a photolithography method; and the like. In the photolithography method, a vacuum facility having a high-precision alignment mechanism and the like are not required, unlike the method for depositing a material to form a pattern. Therefore, an EL element can be manufactured by the photolithography method in a relatively easy manner and at low cost. Compared with the method for color-coding materials based on an ink jet scheme to form a pattern, the photolithography method is preferable since pre-treatments on a structure for assisting the patterning and on a substrate are not performed in the photolithography method. In addition, the photolithography method is more advantageous in terms of formation of a high-precision pattern than the method for color-coding materials based on an ink jet scheme to form a pattern because of the relationship with delivery accuracy of an ink jet head.
As a method for patterning a light emitting layer by photolithography, an etching method, a lift-off method and the like have been proposed. In the lift-off method, the light emitting layer is not exposed by etching, unlike the etching method. The lift-off method can suppress a reduction in performance of the light emitting layer such as light emission characteristics. To pattern a light emitting layer capable of emitting light of multiple colors in the lift-off method, a light emitting layer portion for emitting light of a first color is patterned. Then, a photoresist layer to be used to pattern a light emitting layer portion for emitting light of a second color is formed thereon. It is, therefore, required that the light emitting layer be not dissolved and dispersed in a photoresist solvent, a developer and the like.
In recent years, a light emitting element having a light emitting layer containing a quantum dot composed of a semiconductor particle has been proposed and developed. The quantum dot is a crystal (approximately several nanometers to several tens nanometers) of semiconductor atoms. Since the quantum dot is the nano-sized crystal, electrons confined in the quantum dot exhibit a discrete energy level without exhibiting a continuous energy band structure. The quantum size effect is therefore remarkable. The quantum confinement effect is higher than that in a bulk crystal having a size larger than the quantum dot. The probability of recombination of excitons can be increased. In the light emitting element having the quantum dot, the frequency of light to be emitted can be adjusted while the configuration of the light emitting layer is not changed. The quantum dot exhibits optical characteristics depending on the size thereof due to the quantum confinement effect. For example, the color of light to be emitted from a quantum dot composed of CdSe can be changed from blue to red by simply changing the size of the quantum dot. Even when the quantum dot has a relative small half width, light is emitted from the quantum dot. The half width of the quantum dot can be set to 30 nanometers or less to ensure that light is emitted from the quantum dot. The quantum dot is therefore suitable as a material of the light emitting layer.
It should be noted that although the quantum dot is also called a nano-crystal, a fine particle, a colloid, or a cluster, a material capable of producing the quantum size effect is regarded as the quantum dot in the present application.
As a method for forming a light emitting layer having such a quantum dot, a spin coating method and a dip coating method are known (refer to JP-A-2005-522005 and JP-A-2006-520077, for example). In each of the spin coating method and the dip coating method, a colloid solution containing a quantum dot having a ligand such as tri-n-octylphosphine oxide (TOPO) attached to the surface thereof is used. Since the ligand is attached to the surface of the quantum dot, the ligand serves to improve stability of dispersion of the quantum dots.
To pattern the light emitting layer formed by means of the quantum dots through the lift-off method, it is required that the light emitting layer be not dissolved and dispersed in a photoresist solvent, developer and the like. The ligand (such as tri-n-octylphosphine oxide) attached to the surface of the quantum dot is soluble in many kinds of solvents. It has been, therefore, difficult to form a photoresist layer (to be used to pattern another light emitting layer) on the patterned light emitting layer.
In the conventional light emitting layer containing the quantum dot having the ligand such as TOPO attached to the surface thereof, the quantum dot is not stable. Therefore, the instability may affect the life characteristics of the light emitting element. When the quantum dot is a phosphorescent material, the life characteristics of the light emitting element are easily affected since the life of the phosphorescent material is longer than the life of a fluorescent material.
It is, therefore, an object of the present invention to provide a method for manufacturing an EL element having excellent life characteristics and having a light emitting layer that contains a quantum dot and can be patterned with high stability by a lift-off method.